The present invention generally relates to a method and apparatus for detecting the presence of mineral seams during coal or ore mining operations.
Many concepts have already been tried, over a period of many years, to improve mining controls to increase the amount of coal, or other mineral, cut by the mining equipment and to decrease the amount of undesirable rock, clay or other material cut by the mining equipment. Many of these concepts involve “guidance” systems that direct or point the miner where to cut, based on predictions or assumptions related to the location of the mineral-rock interface. These predictions or assumptions are typically based on data or information obtained from the experience of the mining equipment from previous cuts.
A typical configuration of a continuous miner is to have a rotary cutter, perhaps three to six feet in diameter, followed by a means to collect the cut material and to transport the material to a means of conveyance. In most instances, the ideal operation is for the cutter to remove all the desirable material from above undesirable material or to remove all undesirable material from above the desirable material. However, almost always there is some desirable material left to cut, or some undesirable material cut with the desirable material. After cutting, the mixed undesirable and desirable material may be further separated in a preparation plant to increase the quality of the material. But the separation process adds significant expense, and with known commercially and economically feasible techniques cannot achieve complete separation and purity of the material. A Rock Avoidance System is designed to provide the controls to help accomplish higher quality cutting, either by providing information to the operator or by automatically controlling the position of the cutter relative to the interface between desirable and undesirable material, or a combination of the two approaches.
One technique employs repetitive cycles. A computer is instructed by the miner operator to perform specific cycles or the control system is programmed to memorize operator actions over a cycle and duplicate them. This technique does not work well because of the high variability of the rock and mineral formations and operational considerations. This technique is particularly ineffective when applied to continuous miners and surface miners, because the miner rides on the floor that has been cut resulting in cutting errors (e.g., leaving an excessive layer of coal on the floor, or cutting excessively down into the rock on the floor) for one cut tending to be amplified for subsequent cuts.
In the case of long-wall mining there is some opportunity to utilize what has been learned on one pass along the mineral face to improve upon cutting strategy for the next pass along the face. One technique utilizes a memory system to log the profiles of the rock face at the floor and roof on one pass and then to use this knowledge to influence the cutting as the cutters pass along the same face, going in the opposite direction. This technique has been of only limited success because the rock face profile on one pass does not exactly reflect the needed rock face profile of the next pass and because there is much variability in the formations and mining operations. Consequently, such equipment and operation are limited in their efficiency in cutting to the rock-coal interface using guidance strategy.
Systems have been developed that are able to detect an interface between coal and rock using sensors aimed at the mineral being cut that are mounted away from the cut surface, such as up on the boom of a cutter drum, or up on structure behind the cutter drum. See U.S. Pat. No. 7,402,804, Geosteering of Solid Mineral Mining Machines, the disclosure of which is incorporated herein by reference. However, for some applications, such as surface mining of lignite coal, the radiation emitted by the undesirable material, typically clay, may be weak and difficult to detect by a system described above that is mounted away from the cut surface, such as up on the boom of a cutting drum, or up on structure behind the cutting drum. And on some surface miners, there is insufficient structure to mount a detector system aimed at the mineral being cut.
For a manually controlled approach, the operator needs information about the position of the cutter relative to the coal/rock interface. In the absence of a Rock Avoidance System, the operator must rely upon a variety of cues to determine if the cutter is at the interface, above the interface, or below the interface. In principle, there are two general cutting strategies to be employed. One is to emphasize the avoidance of mining rock or clay while trying to mine as much of the desirable mineral as possible. The other is to ensure that all the mineral is mined but also try to take as little undesirable material as possible. It can be argued that if properly implemented, both strategies produce the same result, but considering all the variables and human factors, one strategy tends to produce higher production and the other tends to produce a cleaner mined product. Choosing between these strategies requires some change in the algorithms or signals that are displayed to the operator but involves that same fundamental elements. Therefore, for purposes of describing the operation, an idealized approach will be taken where the objective is to control the cutter to be positioned at the interface.
The following discussion relates to mining lignite, which typically has layers of clay between layers of lignite coal. The operator of a surface miner is typically looking down at the exposed uncut material behind the cutter, and may be using video cameras, to determine the depth that the cutter is going into the clay below the coal, by visually observing the shade or color of the un-mined material behind the mold board. Once into the clay, even when its appearance is different from the coal, it is impossible to determine from the color or shade of the un-mined material how deeply the cutter is going into the clay. One cue to the operator is to visually observe the color and consistency of the material being dumped into a haul truck that follows along at the side of the miner. Whenever the operator sees significant amounts of clay in the product being mined, referred to as the Run of Mine (ROM), he can respond to raise the cutter. By that time the conveyor is already filled with a mixture of clay and coal. In order to assure that the cutter is not going further into the clay than necessary demands that the operator frequently raise the cutter until the color of the material begins to change, indicating that the cutter is then above the coal-to-clay interface. However, at that point, the cutter is leaving coal. Therefore, the operator must immediately lower the cutter to again begin removing all the coal. Given that the foimation is rolling both in the direction of travel and also perpendicular to that direction, the operator is confronted with a very challenging control task. This controlling operation is even more challenging as a result of two additional factors: (1) reaction time and (2) other operator tasks that distract from the cutting task.
Video cameras are sometimes located behind the mold board and behind the cutter, so a given cut surface (or cut floor) location is seen once the miner has moved forward sufficiently to bring the location in the camera's view, which occurs a few seconds after the cutting has been performed. If a cutting error is made, it cannot be known by the operator until after the cameras pass over the material to expose a change in the color, shade, or character of the un-mined material. Then, there is a time delay required for the operator to react and for the machine to respond to an instruction from the operator. A delay factor is unavoidable for a manually controlled machine.
It is the second factor that, in combination with reaction time, produces the greatest effective cutting error. The operator has many other essential tasks to perform. He must frequently observe the position of the truck relative to the conveyor in order to properly load the truck. He must frequently communicate with the driver of the truck, and sometimes with other trucks that are lining up to be in proper position. He must observe the mined material as it goes onto the truck for indications that too much clay is being mined. There are instruments to monitor and controls to use. The most distracting task may be that of guiding the miner, requiring the operator to look down, in the direction of travel, to keep the crawler tracks in the correct position. Another task is to determine if the surface miner is aligned left-to-right with the formation, in the roll axis. There is a practical limit to how rapidly he can direct his viewing between the other tasks and the controlling of the cutter height.
If the operator is trying to make sure that all the coal is mined, there will frequently be at least a small amount of clay in the ROM. Monitoring ROM quality necessitates the operator to confirm visually that there is a small amount of clay in the coal but to ensure that the amount of clay in the ROM is not excessive. If there is no clay in the ROM, the operator must assume that the cutter is above the interface and is therefore leaving some coal un-mined. He may, depending upon other cues, then immediately lower the cutter without overreacting, considering his reaction time and that of the machine and the variability of the seam interface. This quality monitoring and controlling task requires that the operator be carefully observing the mined product, uninterrupted. Given that the coal being loaded into the truck is the result of cutting actions many feet behind the current location of the cutter, a significant delay in feedback in evaluating quality results, which in turn adds back to the reaction time and delays discussed earlier. When the floor cannot be observed because other tasks are being performed, cutting errors are likely, especially since the cutter must be frequently raised up to the interface in order to assure that it is not going deeply into the clay. As a result, it becomes clear why the operators report that they must rely upon observing the quality of the ROM, even though that is much too late to accomplish cutting with the desired accuracy.
In some instances, the clay under the coal is the same color and shade as the coal, so that there are essentially no visual cues at or around the interface, which is the target. In such instances, the lack of visual cues makes it much more difficult to approximate the ideal cut of all desirable material and no undesirable material.
The above discussion is directed to making the final cut to remove coal from above the coal-to-clay interface. Use of the surface miner to clean clay from the top of a mineable coal seam is also highly desirable. Tasks to be performed by the operator during the cleaning phase are similar to those of the cutting phase. Some of the same type challenges are present. If the priority for the operator is implementing the Full Seam Extraction strategy, the requirement is to leave all the coal after removing as much clay a possible. But, there is no way to determine the thickness of the layer of clay over the coal, based solely upon its appearance, either by direct observation or in the video cameras. A thick layer will have the same appearance as a thin layer. Only when the coal under the clay begins to be exposed can the operator determine that the cutter is at the interface. Therefore, in order to not leave too much clay, he must frequently lower the cutter enough to reach the coal so that the difference in color or shade can be seen. However, when this happens, some coal is being removed with the clay that is to be discarded. So, it is important to quickly raise the cutter again. Accomplishing this task to the precision desired would require that the operator be able to observe the mined interface almost continuously, which was shown in the above discussion to be practically impossible.
Thus, a need exists to provide a system that is able to detect an interface between a layer of lignite coal and a layer of clay to help avoid cutting the lignite layer and clay layer at the same time. And there is a clear need to be able to position the cutter as close as possible to that interface in order to remove all the coal without removing any of the clay under the coal.